The present trend towards miniaturization of mobile communication devices demands for smaller electric and electro-acoustic components. BAW (BAW=bulk acoustic wave) components can be used in RF filters, e.g., in duplexers in front-end modules of mobile communication devices. A duplexer usually comprises a TX (transmission) and an RX (reception) filter. The TX filter and the RX filter are band-pass filters with adjacent but different pass bands. An important factor determining the pass band of a BAW band-pass filter is the thickness of piezoelectric material arranged between two electrode layers of a resonator of the filter and the mass loading of a resonator.
Conventional piezoelectric materials are LiTaO3 (Lithiumtantalate), LiNbO3 (Lithiumniobate), quartz or AlN (aluminium nitride). It is of advantage if the piezoelectric material used for improved MEMS components allows a higher electroacoustic coupling coefficient κ2 or has higher piezoelectric coefficients compared to conventional materials. However, handling such materials is more difficult, and novel methods and processing steps are needed to obtain improved components with unconventional piezoelectric materials such as Sc (Scandium)-doped AlN due to the different physical and chemical properties of Sc-doped AlN.
One type of conventional BAW duplexers has different piezoelectric material thicknesses for the TX filter and for the RX filter. Accordingly, the two filters are manufactured by means of different processes and on different carrier chips.
Another type of conventional BAW duplexers has additional mass, e. g. additional layers on the upper electrode, deposited on selected resonators to reduce their respective resonance frequency.
The manufacturing methods for both types are relatively complex, expensive and susceptible to errors. In particular, conventional duplexers have TX and RX filters on different carrier chips, which is contrary to the miniaturization efforts.